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One-Pot Tandem Diels-Alder/Nazarov Reactions to Generate Advanced Tricyclic Intermediates

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One-Pot Tandem Diels-Alder/Nazarov Reactions to Generate Advanced Tricyclic Intermediates University of Nevada, Reno One-pot Tandem Diels-Alder/Nazarov Reactions to Generate Advanced Tricyclic Intermediates A dissertation submitted in partial fullfilment of the Requirements for the degree of Doctor of Philosophy in Chemistry by Rachael A. Carmichael Dr. Wesley. A. Chalifoux/Dissertation Advisor August 2017 Copyright by Rachael A. Carmichael 2017 All Rights Reserved THE GRADUATE SCHOOL We recommend that the dissertation prepared under our supervision by RACHAEL A. CARMICHAEL Entitled One-Pot Tandem Diels-Alder/Nazarov Reaction To Generate Advanced Tricyclic Intermediates be accepted in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Wesley A. Chalifoux, Ph.D., Advisor Thomas Bell, Ph.D., Committee Member Vincent Catalano, Ph.D., Committee Member Angela Smilanich, Ph.D., Committee Member Christine Cremo, Ph.D., Graduate School Representative David W. Zeh, Ph. D., Dean, Graduate School August, 2017 i Abstract One-pot multi-component reactions represent efficacious strategies to rapidly obtain complex intermediates applicable in the synthesis of therapeutically germane compounds. Nazarov reactions of aryl vinyl ketones are viewed as important steps in total synthesis and contribute to expanding the chemist’s synthetic toolbox. Currently, activated substrates or photochemical means are necessary to effect this transformation because the energy barrier is predicted to be relatively high. The stabilizing effect of silyl substituents beta to a carbocation intermediate has been well documented. a) We hypothesized that incorporation of a silyl group into the ynone starting materials may stabilize the reactive intermediates, enabling the aryl Nazarov reaction to proceed even with unactivated substrates. The use of aryl silyl ynones proved successful and as a consequence our group developed a tandem process utilizing the Diels-Alder and Nazarov reactions of aryl ynones to generate carbo- and heterocyclic fused ring systems in good yields. The tandem reactions proceed under Lewis acidic conditions to generate three new carbon-carbon bonds, a quaternary carbon and two stereogenic centers. In order to access a wider range of pharmaceutically important compounds through concise routes, a more versatile technology is needed. The use of diynones as relatively high energy starting materials facilitates multiple carbon-carbon bond formations in a one-pot reaction. With respect to unsymmetrical diynones, regiocontrol of the double bond is accomplished through silane elimination rather than loss of hydrogen, ensuring the formation of one major product. Furthermore, the Diels-Alder cycloaddition occurs preferentially on the silyl substituted ii alkyne at low temperatures, allowing for a “timed” double Diels-Alder reaction in which two different dienes can be added, generating a highly asymmetric product. Such molecular control permits a significant amount of versatility within the method. b) Our lab has designed and successfully executed a highly modifiable multicomponent reaction, initiating from a double Diels-Alder cycloaddition followed by a Nazarov reaction to furnish the [6-5-6] backbone. This method produces three fused rings evolving from the construction of five new carbon-carbon bonds, quaternary or vicinal quaternary carbons, and stereogenic centers in a one-pot reaction. A diverse array of drug-like scaffolds can be rapidly synthesized through these tandem processes providing a high level of stereo- and regiocontrol in the products. iii Dedication I dedicate this dissertation to Michael R. King for his steadfast love and support iv Acknowledgements I would like to sincerely thank my advisor, Dr. Wesley Chalifoux, for always holding the bar high and helping me to become the researcher that I am today. I would like to thank my committee members, Dr. Thomas Bell, Dr. Vincent Catalano, Dr. Christine Cremo, and Dr. Angela Smilanich for their guidance, time, and support. I would also like to thank Dr. Vincent Catalano, Dr. Stephen Spain, and Casey Philbin for instrumentation training and assistance. I would like to thank my lab mates in the Chalifoux lab, particularly Radha Bam and Dr. Wenlong Yang for their help and support. I would also like to thank the faculty, staff, and students in the UNR Chemistry department for helpful discussions and assistance. I would also like to thank my friends and family for always lending an ear and their support when I needed it most. Additionally, I would like to thank Dr. Darcie Smith- Smiley and Mason Smiley for their support and advice and Darcie for editing my writing. Lastly, I would like to thank Michael King for his love and support. v Table of Contents 1 Introduction ................................................................................................................. 1 1.1 Pericyclic Reactions in Synthetic Methodology .................................................. 1 1.2 The Diels-Alder Cycloaddition ............................................................................ 2 1.3 The Nazarov Reaction .......................................................................................... 4 1.4 Tandem or Cascade Reactions ............................................................................. 7 1.5 Conclusion ............................................................................................................ 9 1.6 References ............................................................................................................ 9 2 β‑Silyl-Assisted Tandem Diels−Alder/Nazarov Reaction of 1‑Aryl-3-(trimethylsilyl) Ynones .............................................................................................................................. 11 2.1 Introduction ........................................................................................................ 11 2.2 Nazarov Cyclization of Aryl Substituted and Terminal Ynones........................ 13 2.2.1 Unexpected Formation of Mystery Product ................................................ 23 2.3 Tandem Diels-Alder/Nazarov Reaction of Silyl-substituted Ynones ................ 28 2.3.1 Catalytic Screening for Tandem Diels-Alder/Nazarov Reaction................ 38 2.4 Conclusion .......................................................................................................... 40 2.5 Experimental ...................................................................................................... 40 2.6 References .......................................................................................................... 56 vi 3 Multicomponent Double Diels-Alder/Nazarov Tandem Cyclization of Symmetric Cross-conjugated Diynones to Generate [6-5-6] Tricyclic Products ................................ 59 3.1 Introduction ........................................................................................................ 59 3.2 Reactions of Diynones ....................................................................................... 62 3.3 Preliminary Results: Double Diels-Alder Proof of Concept .............................. 63 3.4 One-pot Double Diels-Alder/Nazarov Tandem Reaction of Diynones ............. 66 3.4.1 Double Diels-Alder/Nazarov of Symmetrically Substituted Silyl Diynones ................................................................................................................... 71 3.4.2 Double Diels-Alder/Nazarov Scope for Symmetrical Diynones ................ 73 3.4.3 Double Diels-Alder/Nazarov of Symmetrically Substituted Alkyl/Aryl Diynones ................................................................................................................... 75 3.5 Enantioselective Efforts ..................................................................................... 78 3.6 Investigations into Timed Diene Addition Reactions ........................................ 84 3.7 Scalability of Double Diels-Alder/Nazarov Tandem Reaction .......................... 86 3.8 Conclusion .......................................................................................................... 86 3.9 Experimental ...................................................................................................... 87 3.10 References .................................................................................................... 102 4 One-pot Synthesis of [6-5-6] Tricyclic Products via a Double Diels-Alder/Nazarov Tandem Reaction of Unsymmetrically-substituted Cross-conjugated Diynones ........... 105 4.1 Introduction ...................................................................................................... 105 vii 4.2 Double Diels-Alder/Nazarov Tandem Reaction of Diynones .......................... 106 4.3 Triethyl Silyl versus Trimethyl Silyl Unsymmetrical Diynones ...................... 109 4.4 Optimization Attempts for the Unsymmetrical Scope ..................................... 112 4.5 Scope of Unsymmetrically-substituted Trimethyl Silyl Diynones .................. 117 4.6 Multi-diene Reactions ...................................................................................... 119 4.6.1 Synthetic Efforts Towards Biologically Active Compounds.................... 124 4.7 Conclusions ...................................................................................................... 125 4.8 Experimental .................................................................................................... 125 4.9 References .......................................................................................................
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